mainstream theory, proposed in 1977 by Thomas Kirkwood, presumes that the body must budget the resources available to it. The body uses resources derived from the environment for metabolism, for reproduction, and for repair and maintenance, and the body must compromise when there is a finite supply of resources. The theory states that this compromise causes the body to reallocate energy to the repair function that causes the body to gradually deteriorate with age. A caveat to this theory suggests that this reallocation of energy is based on time instead of limiting resources. This concept focuses on the evolutionary pressure to reproduce in a set, optimal time period that is dictated by age and ecological niche. The way that this is successful is through the allocation of time and energy in damage repair at the cellular level resulting in an accumulation of damage and a decreased lifespan relative to organisms with longer gestation. This concept stems from a comparative analysis of genomic stability in mammalian cells. One opposing argument is based on the effect of caloric restriction, which lengthens life. However, dietary restriction has not been shown to increase lifetime reproductive success (fitness), because when food availability is lower, reproductive output is also lower. Moreover, calories are not the only resource of possibly limited supply to an organism that could have an effect on multiple dimensions of fitness. DNA damage/error theory Just like DNA mutation and expression have phenotypic effects on organisms, DNA damage and mutation accumulation also have phenotypic consequences in older humans. Damage to macromolecules such as DNA, RNA, and proteins along with the deterioration of tissues and organs are the basis of aging. Species-specific rates of aging are due to deleterious changes which manifest after the reproductive phase. "Mitochondrial DNA (mtDNA) regulates cellular metabolism, apoptosis and oxidative stress control". Damage to mtDNA is therefore another contributing factor to phenotypes related to aging. Neurodegeneration and cancer are two factors that manifest with DNA damage; therefore, we need to understand the change in the association between DNA damage and DNA repair as we age in order to be aware of age-related diseases and develop lifestyles that could possibly promote a healthy life span. The DNA damage theory of aging postulates that DNA damage is ubiquitous in the biological world and is the primary cause of ageing. The theory is based on the idea that ageing occurs over time due